A general problem with probes as indicated above, for instance, dissolved oxygen measuring probes (DO probes) is that probe failure caused by, for example, fouling of the membrane may be interpreted as a change in the concentration of the substance to be measured.
Normally this problem is solved by calibrating the probe on a regular time basis. However this solution is not without problems, because by periodically calibrating the probe, the calibration can be either too late or too early. When the calibration is done too early it costs too much working-hours and if the calibration is too late the probe had produced false values during an unknown period.